The invention relates to a superconductive high-current switch.
Switches usable for switching high power, are, as a rule, of a complicated construction as specific measures need to be taken to counteract the formation of electric arcs and to eliminate arcs formed, respectively. A useful quantity for assessing a power switch is the switching capacity related to a unit of volume (current * voltage/component volume). With the conventional switches, this value is relatively small, due to construction expenditures.
What is disadvantageous in the conventional high-current switches is, furthermore, the relatively long switching time that usually is indicated for a complete switching cycle consisting of activation and deactivation of an electric circuit.
For solving the problems indicated hereinbefore, the applicant has already suggested a superconductive high-current switch (WO89/05044) in which plate-shaped insulating carriers having a high-temperature superconductive material vapor-deposited therein are arranged in a housing in stacked manner.
High-temperature superconductors are in particular such conductors having a transition temperature of more than approx. 80xc2x0 K. Typical high-temperature superconductive materials are ABa3CuO7 (wherein A=Y, La, Nd, Sm, Eu, Gd, Ho, Xr, Lu) as well as Y1.2Ba0.8CuO4.
The switch function of a superconductive high-current switch consists in the transition between the superconducting and normally conducting state, and vice versa. In the superconducting state, the switch is closed, i.e. current flows without electric resistance through the switch unit. By suitable cooling of the switch, the latter is brought to the superconducting state. For opening the switch, there are various mechanisms possible, for example heating of the superconductive material by electrical heating, by laser beam etc. Further possibilities are the application of external magnetic fields and the introduction of an additional current phase. As regards to further details, reference is made to the above indicated WO publication.
In the normally conducting state of the superconductive material, a considerable electrical resistance is present between the input terminal and the output terminal of the switch. Due to the construction of the known superconductive high-current switch, the dielectric strength is relatively low. In addition, to the material data of the superconductor in the normally conducting and superconducting states, the dielectric strength of the assembly determines the attainable switching capacity. Consequently, the low dielectric strength of course means a relatively low upper limit for the switching capacity.
It is the object of the invention to provide a superconductive high-current switch which distinguishes itself by a relatively high dielectric strength along with a high switching capacity per component unit of volume. In particular, a switch is to be provided for high powers, having switching capacities of preferably more than 1 MVA, in particular more than 1 GVA. particular, a switch is to be provided for high powers, having switching capacities of preferably more than 1 MVA, in particular more than 1 GVA.
With a superconductive high-current switch, this object is met according to the invention by the following features:
a) there is provided at least one layer having a winding of a superconductive, in particular high-temperature superconductive, current conductor section so as to form a high-current switch unit;
b) within each winding layer, the line routing is such that the current to be switched flows in opposite directions in radially adjacent current conductor sections; and
c) there is provided a switching control means capable of effecting in the current conductor section a change between the conditions xe2x80x9csuperconductingxe2x80x9d and xe2x80x9cnot superconductingxe2x80x9d.
In the superconductive high-current switch according to the invention, the switching control means actively brings the current conductor sectionsxe2x80x94at least in partxe2x80x94from the state present at the particular moment (superconducting/normally conducting) to the other state (normally conducting/superconducting) in order to thus xe2x80x9copenxe2x80x9d and xe2x80x9cclosexe2x80x9d the switch, respectively. The current conductor section in the winding layerxe2x80x94electrically constitutes a continuous current conductor between an input terminal and an output terminal of the switch. As will still be elucidated in more detail further below, a specific embodiment of the invention is provided with a single, continuous current conductor, e.g. in the form of a pre-insulated wire. In a preferred embodiment, several layers with one winding means each are lined up adjacent each other in the direction of the winding axis. What is essential in this respect is that the voltage present between switch input and output is distributed to several layers in axial direction of the assembly. Due to this, the voltage to be switched is distributed to several layers so that very high dielectric strength is obtained. In the superconducting state, there are considerable currents flowing. By means of the switch according to the invention, it is thus possible to obtain high switching capacities. All embodiments of the invention distinguish themselves by a relatively simple, but definitely very compact construction, so that the electric switching capacity related to the unit of volume of the switch is considerable, which makes the switch according to the invention superior as compared to conventional switches.
An additional specific feature of a power switch is the switching speed. According to the invention, the current direction within each layer in the individual current conductor sections extends such that the current flow direction is opposite in adjacent current conductors. Due to the fact that each winding means of a conductor naturally has a certain inductance, the switching operation in a switch is impaired by the energy stored in the inductance. Due to the opposite current flow direction according to the invention, a low-inductance construction is obtained, so that the switching speed is thus high.
In a specific embodiment of the invention, the individual lined-up layers of the high-current switch according to the invention have the same or at least a similar structure, in particular with current conductor sections adjacent in axial direction having virtually the same radial. distance from the winding axis. In such axially adjacent current conductors of adjacent layers, the current flow directions in a less expedient embodiment may be in the same direction, but preferably the current flow directions. are also opposite in the axially adjacent current conductor sections as well. When the current flow directions are opposite not only in radially adjacent but also in axially adjacent current conductor sections, the particularly, advantageous properties of the high-current switch according to the invention, as elucidated hereinbefore specifically for one winding layer each, are still enhanced. This holds in particular for the inner mechanical forces between the current-conducting parts, which remain very low despite the compact construction and thus permit the conduction of higher currents.
The effects and advantages of the invention elucidated in the following concern various embodiments relating to the design of the current conductor sections, the layer structure, the entire structure of the switch with its specific electrical and mechanical features and the construction and operation of the switch in consideration of the superconductivity of the current conductor material.
In order to effectively exploit the high dielectric strength, the high switching capacity and the high switching speed, there should be provided a minimum number of layers, which possibly should be an even number of layers for reasons of symmetry. Preferred are at least four layers.
Superconductors are available in the market in various embodiments, e.g. as pre-insulated wire. In a preferred embodiment of the invention, all current conductor sections are formed of windable wire, preferably the entire switch construction consisting of one continuous windable wire, with the winding in the individual layers and the relation of the windings in adjacent layers being selected such that the above-elucidated opposite current flow direction in adjacent current conductor sections is achieved. The specific winding geometry will still be dealt with further below.
The windable wire may consist of filament wire with a resistance matrix. In such a wire, numerous very thin filaments are embedded in the matrix of resistance material. An alternative to the windable wire is a cable formed of filament wires, e.g. a braided cable. With such a windable wire, relatively narrow bending radii can be realized, without the wire being damaged in the winding operation. The intact, wound wire may carry the full nominal current so that the switch constituted by the wire does not have resistance in the closed state.
The current conductor sections may also consist directly of high-temperature superconductive material.
In particular, it is preferred to implement the entire switch construction of one continuous wire. In this case, no additional length will be required for soldering the sections.
For obtaining a good ratio of electric switching capacity and unit, of volume of the switch, it is advantageous to keep the total volume to be provided for insulation and mechanical stabilization of the switch very low. A practical possibility to obtain this effect consists in directly winding pre-insulated current conductors, dispensing with any structure material. With this embodiment, the merely pre-insulated current conductors are wound into the desired shape without requiring additional specific insulating measures between the individual windings. The desired dielectric strength is achieved by a correspondingly high number of layers and a corresponding length of the switch, respectively, so that the pre-insulation is. sufficient to prevent voltage breakdowns. In a specific embodiment, the dielectric strength is obtained by subsequent impregnation with an insulation casting compound, e.g. epoxy resin. This process can be carried out in layer-wise manner or on the complete unit. It is possible to provide the individual wound layers of pre-insulated wire with an insulation layer in layer-wise manner, so that enhanced insulation is obtained between the layers, in order to comply with the voltage between two axially adjacent current conductor sections which is somewhat higher than the voltage between two radially adjacent current conductor sections.
Preferably the entire constructional unit of the high-current switch is provided with an insulation layer which, in a specific embodiment, also serves as a mechanical stabilization.
Instead of winding only pre-insulated conductors in a radial direction in order to obtain the individual layers, it is also possible to make use of current conductor sections having a sheathing material. The sheathing material of the current conductor sections servesas insulation both between radially and between axially adjacent current conductors. The criterion for designing the dielectric strength of the sheathing material then is the highest voltage occurring between two layers. It is preferred to fill the entire high-current switch with suitable casting material and to obtain mechanical stabilization of the unit with the aid of this casting compound. The specific embodiments of the invention elucidated hereinbefore relate to the use of a windable wire. As an alternative, it is however also possible to use a structure material which predetermines the subsequent shape of the winding, by introducing the windings into the structure material. The structure material may be, e.g., an insulating material body having openings in accordance with the subsequent windings. When such a structure material is used, a specific embodiment of the invention provides that ceramic high-temperature superconductive material is filled as a preliminary product into the openings of the structure material, whereupon the superconductive material is suitably heat-treated, e.g. sintered or textured, in the molten form. The superconductive material can also be applied in thin layers by vapor deposition.
The structure material can not only be used with superconductive material to be heat-treated, but also with windable wires. In any case the structure material may serve as insulating material and also as material for mechanical stabilization of the component. While the afore-mentioned features of the high-current switch according to the invention relate specifically to the design of the current-conducting windings proper, the following discussion shall deal in more detail with the means through which the windings are transferred from the superconducting state to the normally conducting statexe2x80x94and vice versa.
For transferring from the superconducting to the normally conducting state, preferably an integrated heating means is employed that is arranged between axially adjacent layers. A heating means may be integrated between each adjacent layer, but it is also possible to integrate heating means between every two or three or. more layers that are combined as a block.
When the high-current switch according to the invention is closed, i.e. when the current-conductor sections are in the superconducting state, all current-conductor sections are cooled to such an extent that they conduct current without resistance below the transition temperature. For opening the switch, it is possible to activate e.g. the above-described heating means so that the temperature at least of individual portions of the current conductor sections increases beyond the transition temperature. The state of superconductivity thus breaks down in the portions of increased temperature. It is possible to design the heating means such that virtually all current conductor sections change to the non-conducting state everywhere at the same time. In a preferred embodiment of the invention, however, it is sufficient if a temperature increase takes place only partially. Upon transition to the normally conducting state, the respective current conductor sections generate sufficient dissipation heat for heating the so far not yet normally conducting, adjacent current conductor sections to such an extent that the temperature thereof increases beyond the transition temperature. This transition takes place relatively rapidly, so that the entire current conductor length has a very high electric resistance within a short period of time, which corresponds to the opened state of the switch.
For transferring the high-current switch from the normally conducting to the superconducting state, the current conductor sections must be cooled down to such an extent that the temperature thereof is below the transition temperature. This takes place normally by the cryogenic cooling medium necessary for obtaining superconductivity. In particularly preferred mannerxe2x80x94similar to the heating means elucidated hereinbeforexe2x80x94cooling means are provided between the individual layers of the switch. In a specific embodiment, thermoelectric power elements are arranged between the individual layers of the switch, such as e.g. specific Peltier elements capable of operating at the low temperatures typical for superconductors. These thermoelectric power elements can be operated both as heating means and as cooling means, as known in principle from the prior art.
In a further embodiment of the invention, an externally generated magnetic field and/or a short, high additional current pulse passed through the winding serves for transfer from the superconducting to the normally conducting state. This additional current pulse has a duration of less than 10 xcexcs. It may also be generated by short-circuiting the load. Triggering by an additional current pulse is particularly advantageous if the deactivation operation is to be extremely fast and the voltage drop is to be formed immediately over the entire conductor length.
The above-described mechanism of exploitation of the self-heating effect can be used here as well.
Depending on the particular application, it may be desired that the current flowing through the load to be switched is limited in its amount. In such a specific case, a particular embodiment of the invention may be designed such that, when a threshold value of the rising current is exceeded, this causes a spontaneous transition of the current conductor sections from the superconducting to the normally conducting state. When sufficient cooling of the superconducting material is reached again thereafter (e.g. by continuously activated cooling means), the superconducting state is resumed, so that an in total periodic activation and deactivation can be realized by the high-current switch according to the invention.